1. Field of the Invention
The invention relates to the control of a hybrid electric vehicle using a modified engine power demand at zero and near-zero vehicle speeds.
2. Background Discussion
A parallel hybrid electric vehicle powertrain uses two sources of power. A first source is an internal combustion engine, and a second is an electric motor, a generator and a battery. A parallel hybrid electric vehicle design of this type is disclosed, for example, in co-pending U.S. patent application Ser. No. 10/248,886, filed Feb. 27, 2003, which is owned by the assignee of the present invention.
In the powertrain configuration of the co-pending patent application, a combination of an engine and generator uses a planetary gearset to establish a mechanical torque flow path and a separate electromechanical torque flow path to vehicle traction wheels. The battery is an energy storing device for the generator and the motor.
Engine power is divided into two power flow paths by controlling the generator speed. A mechanical path from the engine to the planetary gearset carrier (Teωe), to the planetary gearset ring gear, to a countershaft gearing output (Tsωs) is established in addition to an electro-mechanical mechanical power flow path (Tgωg to Tmωm), which extends from the engine to the generator to the motor and to the countershaft gearing. Engine power is divided and is controlled by the generator speed, which implies that the generator speed can be decoupled from the vehicle speed. This mode of operation is called is a “positive split.”
Because of the mechanical properties of the planetary gearset, the generator can input power to the planetary gearset to drive the vehicle. This mode of operation is called “negative split.” This combination of a generator, a motor and a planetary gearset thus can be considered to be an electromechanical transmission with CVT (continuously variable transmission) characteristics.
A generator brake can be activated so that engine output power then is transmitted with a fixed gear ratio to the torque output side of the powertrain through the mechanical power flow path only. This first power source can only effect forward propulsion of the vehicle since there is no reverse gear. The engine requires either generator control or application of the generator brake to transmit output power to the powertrain output for forward drive.
When the second power source is activated, the electric motor draws power from the battery and effects vehicle propulsion independently of the engine for both forward drive and reverse drive. In addition, the generator can draw power from the battery and drive against a one-way clutch on the engine power output shaft to propel the vehicle forward. The generator can propel the vehicle independently of the engine when that is necessary. This mode of operation is called “generator drive mode.”
In order to integrate the two power sources to work together seamlessly to meet the driver's torque demand without exceeding the powertrain system limits, including battery limits, while optimizing total powertrain system efficiency and performance, coordinated control of the two power sources must be achieved. The powertrain of the co-pending patent application has a hierarchical vehicle system controller (VSC) that performs this coordinated control. Under normal powertrain operating conditions the VSC interprets the driver's demands (e.g., PRNDL selection and acceleration or deceleration demand) and then determines when and how much torque each power source needs to provide to meet the driver's demands and to achieve specified vehicle performance with respect to fuel economy, emissions, drivability, etc. The VSC controller determines the operating point on the characteristic torque-speed curve for the engine.
The coordinated control provided by the VSC for the two power sources is needed to meet driver demand, without exceeding the system power limits, to optimize the total system efficiency and performance. In addition power control is needed because the two power sources (i.e. engine and battery) in the powertrain system cannot both be expressed in terms of torque. In order to control power, therefore, it is necessary to convert driver torque demand to a power demand. The power demand will dictate how the powertrain system is being operated (e.g., how much power the engine should produce).
Difficulty in converting the driver's torque demand to a power demand arises when the vehicle speed or the motor speed is zero or near zero. The vehicle speed is proportional to motor speed. A power demand during a full accelerator pedal launch from a standing start effectively would be zero at the beginning of the launch, notwithstanding the fact that the driver is demanding full torque, if the motor speed were not properly mapped and made available to the controller to compute needed engine power. In addition, if the motor speed were mapped to a value that is too high (other than zero), that would result in a power demand that is too high, which would make the engine produce excess power. This excess power could exceed the battery charge limit, which may cause the vehicle to shut down.
When the vehicle is rolling backward and the driver is demanding a full accelerator pedal launch in a forward direction, improper motor speed mapping may result in a negative power demand even after the vehicle overcomes the rolling backward condition and moves in a forward direction. This negative power demand could adversely affect any engine power command by the controller. Without the engine outputting power, the vehicle acceleration performance could tend to be degraded.